Migrate async tests to strong

tests/lib_strong/async/zone_empty_description2_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'dart:async';
+
+testEmptyZoneSpecification() {
+  Expect.identical(Zone.ROOT, Zone.current);
+  Zone forked = Zone.current.fork();
+  Expect.isFalse(identical(Zone.ROOT, forked));
+
+  asyncStart();
+  bool timerDidRun = false;
+  forked.createTimer(const Duration(milliseconds: 20), () {
+    // The createTimer function on the Zone binds the closures.
+    Expect.identical(forked, Zone.current);
+    timerDidRun = true;
+    asyncEnd();
+  });
+  Expect.identical(Zone.ROOT, Zone.current);
+
+  asyncStart();
+  int periodicTimerCount = 0;
+  forked.createPeriodicTimer(const Duration(milliseconds: 20), (Timer timer) {
+    periodicTimerCount++;
+    if (periodicTimerCount == 4) {
+      timer.cancel();
+      asyncEnd();
+    }
+    // The createPeriodicTimer function on the Zone binds the closures.
+    Expect.identical(forked, Zone.current);
+  });
+  Expect.identical(Zone.ROOT, Zone.current);
+}
+
+main() {
+  testEmptyZoneSpecification();
+}